The invention relates to a process for preparing a solution which comprises urea urethanes and is effective as a thixotropic agent, which involves reacting monohydroxy compounds with an excess of toluene diisocyanate, removing the unreacted portion of the toluene diisocyanate from the reaction mixture, and further reacting the resultant monoisocyanate adducts with diamines in an aprotic solvent in the presence of a lithium salt to give urea urethanes. The invention additionally relates to the use of the solution to receive thixotropic coating compositions.
In order to control the rheology of liquid coating systems, predominantly organically modified bentonites, silicas, hydrogenated castor oil and polyamide waxes are used. A disadvantage of these substances is that they are mostly dry solids which have to be brought into the form of a semi-finished product using solvents and shear forces, and incorporated into the liquid coating system under careful temperature control. Failure to observe such temperatures results in crystallites in the finished coating system, which may lead to defects in the coating.
The general disadvantage of these presently used theological auxiliaries is that they lead to turbidities and haze in clear, transparent coatings. Moreover, handling dry pulverulent products which give rise to dusts in the course of processing is undesirable.
Different solutions for rheology control have been described in European Patent Application EP-A-0 198 519. There, an isocyanate is reacted with an amine in the presence of solutions of film-forming resin to give a urea which forms acicular crystals in very finely disperse form. The film-forming binders thus modified are sold as rheology-controlling and anti-sagging binders, referred to as sag control agents. The disadvantage of these products is that they are always bound to the binders in which they were prepared, and there is no possibility of subsequent, universal correction of finished coating compositions.
European Patent EP-B-0 006 252 describes a process for preparing a thixotropic agent that removes some of the abovementioned disadvantages, describing urea urethanes which are prepared by reaction of isocyanate adducts with polyamines in aprotic solvents in the presence of lithium salts. The products prepared, however, have two significant disadvantages. Firstly, these thixotropic agents are characterized by an undefined structure owing to the preparation process. Although monoisocyanate adducts are described, the actual products are in fact not monoadducts at all, as is clearly evident from the example, but rather mixtures of different adducts. In the process described, one mole of a diisocyanate is first reacted with one mole of a monoalcohol. By this process the desired NCO-functional monoadducts are partially formed, but also non-NCO-functional diadducts, which in the course of subsequent reaction with polyamines in the presence of lithium chloride leads to uncontrolled chain extension of the urea urethane and to polymeric ureas. These products then tend to display precipitation phenomena and are extremely difficult to keep in solution. A further disadvantage of the thixotropic agents prepared by this process can be recognized in the fact that always only monoisocyanate adducts with the same structure are reacted with the diamine. This leads firstly to limited compatibility in the coating systems used, manifested in gel structures or severe turbidities, and secondly to a poorer rheological effectiveness.
It is therefore an object of the present invention to find thixotropic agents which on the one hand have a defined structure and therefore permit relatively high storage stability of the solution thus prepared, over several months, and on the other are characterized by a relatively broad compatibility in binders, thereby permitting reliable use of the products. This is of particular interest in modem coating systems which are relatively polar formulations, such as waterborne coating materials or high-solids systems, for example.
Surprisingly it has been found that this object can be achieved in a process of the type specified at the outset by reacting at least two structurally different monoisocyanate adducts, which differ in their alcohol component, with the diamines to give urea adducts.
The present invention accordingly provides a process in which at least two structurally different monohydroxy compounds of the general structure Rxe2x80x94OH, in which R represents an n-alkyl radical or an iso-alkyl radical having from 4 to 22 carbon atoms, an alkenyl radical having from 3 to 18 carbon atoms, a cycloalkyl radical, an aralkyl radical, or a radical of the formula CmH2m+1(Oxe2x80x94CnH2n)x, CmH2m+1(OOCxe2x80x94CvH2v)x or Zxe2x80x94C6H4(Oxe2x80x94CnH2n)x, in which m is 1-22, n is 2-4, x is v is 4 or 5 and Z is an alkyl radical having from 1 to 12 carbon atoms, are reacted with a from 1.5- to 5-fold excess of toluene diisocyanate to give monoisocyanate adducts of the general structure (I) 
and the unreacted portion of the toluene diisocyanate is removed from the reaction mixture, and the resultant monoisocyanate adducts are reacted with diamines of the formula H2Nxe2x80x94R3xe2x80x94NH2, in which R3 is a radical having the formula xe2x80x94CiH2ixe2x80x94, where i is from about 2 to about 12;xe2x80x94(CpH2pxe2x80x94Oxe2x80x94CpH2p)q, where p is from about 2 to about 4 and q is from about 1 to about 10; 
where R4 is CH3 or hydrogen; 
or mixtures thereof in an aprotic solvent in the presence of a lithium salt to give urea adducts of the general structure (II) 
wherein the radicals R1 and R2 satisfy the conditions for the radical R.
In the process of the invention, the abovementioned diamines may be replaced in whole or in part by paraxylylenediamine of the formula 
Alkyl, alkoxy, alkylene, etc. denote both straight and branched groups; but reference to an individual radical such as xe2x80x9cpropylxe2x80x9d embraces only the straight chain radical, a branched chain isomer such as xe2x80x9cisopropylxe2x80x9d being specifically referred to. Alkylene denotes a divalent straight or branched alkyl group (e.g., methylene (xe2x80x94CH2xe2x80x94) or ethylene (xe2x80x94CH2CH2xe2x80x94)). When alkyl or alkylene can be partially unsaturated, the alkyl chain may comprise one or more (e.g. 1, 2, 3, or 4) double or triple bonds in the chain.
By the process of the invention, the solution which comprises urea urethanes and is active as a thixotropic agent may be obtained in principle by two different routes:
a) on the one hand, it is possible first to mix at least two structurally different alcohols Rxe2x80x94OH (i.e., R1xe2x80x94OH and R2xe2x80x94OH) and then to react the mixture with a from 1.5-to 5-fold excess of toluene diisocyanate. The unreacted portion of the toluene diisocyanate is subsequently removed from the reaction mixture under gentle conditions, in accordance with the prior art, and the resultant mixture of the structurally different monoisocyanate adducts is then reacted with the diamines in an aprotic solvent in the presence of a lithium salt to give urea urethanes of the general structure (II).
b) on the other hand, it is possible first to react at least two structurally different alcohols Rxe2x80x94OH separately from one another with a from 1.5- to 5-fold excess of toluene diisocyanate. The unreacted portion of the toluene diisocyanate is removed from the respective reaction mixture under gentle conditions, in accordance with the prior art, and the resultant, structurally different monoisocyanate adducts are mixed with one another. The resultant mixture of structurally different monoisocyanate adducts is then reacted with the diamines in an aprotic solvent in the presence of a lithium salt to give urea urethanes of the general structure (II).
The molar fraction of the respective monoisocyanate adducts in the mixture of structurally different monoisocyanate adducts is between 20 and 80%, preferably between 35 and 65%, with particular preference between 45 and 55%, the sum of the molar fractions of the monoisocyanate adducts being 100%. The molar excess of tolylene diisocyanate is preferably from 1.5 to 5.0 mol, with particular preference from 2.0 to 4.0 mol.
The solids content of the urea urethane solutions produced is from 5 to 80%, preferably from 20 to 60%, with particular preference from 25 to 50%. The reaction of the monoisocyanate adduct mixtures with the diamine takes place in polar aprotic solvents, such as dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, N-butylpyrrolidone or comparable alkylpyrrolidones, or mixtures thereof, for example.
The fraction of lithium compounds is from 0.2 to 2 mol, preferably from 0.5 to 1.5 mol, with particular preference from 0.75 to 1.25 mol of lithium based on the amine equivalent of the diamine used.
Particularly advantageous is the use of LiNO3 rather than LiCl, since chloride ions have adverse effects in coating systems and may promote the corrosion of the metallic substrates to which the thixotropic coating systems are applied.
The alcohols Rxe2x80x94OH used to prepare the monoisocyanate adducts preferably comprise linear or branched primary alcohols which may be saturated or unsaturated, such as n-butanol, 2-ethylhexanol, isotridecyl alcohol, Guerbet alcohols of chain length C10 to C20, oleyl alcohol, linoleyl alcohol, lauryl alcohol, stearyl alcohol, for example, but cycloaliphatic alcohols, such as cyclohexanol or its alkyl-substituted derivatives, for example, and, additionally, aromatically substituted alkanols such as benzyl alcohol, are also suitable.
Particularly suitable for adjusting the polarity are the alkoxylated derivatives of the abovementioned alcohols, in which case lower alcohols such as methanol or allyl alcohol, for example, may also be used as starting components for the alkoxylation. The products thus prepared include preferably ethylene oxide and/or propylene oxide units in the chain and may contain these units in alternation or in blocks. For the alkoxylation it is also possible to use aromatic alcohols such as phenols or alkyl-phenols, for example, as starting components. Preferred alkoxylated alcohols are methoxypolyethylene glycol 500, methoxypolyethylene glycol 350 and triethylene glycol monobutyl ether (butyl triglycol).
In order to adjust the compatibility of the urea urethanes of the invention to modern binder systems, it is also possible to incorporate ester groups or polyester groups into the alcohol component, for example by addition reaction of lactones, such as xcex5-caprolactone, for example, with the abovementioned alcohols or alcohol alkoxylates, or by the use of hydroxy-functional (meth)acrylates.
The diisocyanates which are used to form the monoisocyanate adducts substantially comprise tolylene diisocyanates in the known and customary isomer distribution-in the course of the distillation of the excess fractions of diisocyanate, shifts may occur in the proportion of isomers, so that higher proportions of 2,6-toluene diisocyanate than commonly available commercially may also be formed. These distillates may be used again in the preparation of further monoadducts. Preference is given to toluene diisocyanate isomers having a 2,4-isomer fraction of from 50 to 100%.
The diamines of the formula H2Nxe2x80x94R3xe2x80x94NH2 that substantially comprise linear diamines of chain length C2 to C12 which may be straight-chain or branched, such as 1,3-propanediamine, hexamethylenediamine, octamethylenediamine, diaminododecane or neopentanediamine, for example. Likewise suitable are cyclic diamines such as 4,4xe2x80x2-diaminodicyclohexylmethane or 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diamino-dicyclohexylmethane, for example. Particular preference is given to aromatic-aliphatic diamines such as meta-xylylenediamine or para-xylylenediamine, for example. The diamines may also be used as a mixture in order to form the urea, since by this means the crystallization tendency of the urea urethane in solution is reduced.
The urea urethanes prepared by the process of the invention contain neither free isocyanate nor free amino groups. Accordingly, they are physiologically acceptable. Moreover, there are no adverse side-reactions with binders or fillers.
The storage stability of the urea urethane solutions prepared in this way is extremely high and at normal storage temperatures is easily 6 months or more. Furthermore, the urea urethane solutions possess broad compatibility in binders and therefore permit reliable use of the thixotropic agents.
The present invention additionally provides for the use of the urea urethane solution prepared by the process of the invention to render coating compositions thixotropic. The coating compositions comprise, preferably, aqueous, solvent-borne and solvent-free coating materials, PVC plastisols, epoxy-based coatings, and those based on unsaturated polyester resins.
The main features of the process of the invention are illustrated by the following working examples.